FIG. 6 is a diagram showing an example of a formulation of a conventional transmitter in case of having only one transmitting circuit. In FIG. 6, the conventional transmitter comprises a GCA (Gain Control Amplifier) 11 which makes up a transmitting circuit 1, a PA (Power Amplifier) 12 which makes up the transmitting circuit 1, an RF (Radio Frequency) coupler 13, a detector circuit 14; a reference voltage generator 15, a comparator 16, and an integrating circuit 17. FIG. 7 is a diagram showing a structure of the detector circuit 14 in FIG. 6. The detector circuit 14 comprises a diode unit 14-1 and a LPF (Low-Pass Filter) 14-2. The detector circuit 14 may comprise a detector IC instead of the diode unit.
The GCA 11 is an amplifier whose gain can be variably controlled by an external analog voltage. The PA 12 is an amplifier disposed at the final stage of the transmitting circuit; and it has a constant gain. The RF coupler 13 branches a radio frequency signal power output from the PA 12 and supplies part of the power to the detector circuit 14. The detector circuit 14 detects the branched radio frequency signal power and outputs it as a detection signal. The detection voltage should be higher by a certain level than the noise floor to improve its error estimation accuracy. The comparator 16 compares the detection voltage with a reference voltage VRef of a target transmission power generated by the reference voltage generator 15 and outputs a difference voltage. The difference voltage is integrated by the integrating circuit 17 and input to the GCA 11 as a feedback; therefore, the gain of the GCA 11 is variably controlled.
A high accuracy in the power detection is required for the detector circuit 14, because the circuit 14 is used for the purpose of controlling the output power of the transmitting circuit accurately toward a predetermined value. Therefore, the output voltage (detection voltage) of the detector circuit 14 should be high enough above the noise floor level.
In general, if there is provided a plurality of transmitting circuits each deals with different frequency bands, a detector circuit is equipped for each of the transmitting circuits. FIG. 8 is a diagram showing a formulation of a conventional transmitter with two transmitting circuits, wherein an equivalent component with that in FIG. 6 is indicated by an identical reference numeral. As well as the components in FIG. 6, the transmitter shown in FIG. 8 further comprises a GCA 21 which makes up a transmitting circuit 2, a PA 22 which makes up the transmitting circuit 2, an RF coupler 23 for branching a radio frequency signal power output from the PA 22 and supplying part of the power to a detector circuit 24, and a detector circuit 24 for detecting the branched radio frequency signal power output. A structure of the detector circuit 24 is similar to that in the detector circuit 14 shown in FIG. 7.
The transmitting circuit 1 and the transmitting circuit 2 are operated exclusively to each other; while one is in operation, the other is stopped. For example, the transmitting circuit 1 deals with signals of a radio frequency band (for example, 2 GHz band); and the transmitting circuit 2 deals with signals of a low frequency band (for example, 800 MHz band).
In this case, since the detector circuit in one transmitting circuit is independent from the detector circuit in the other transmitting circuit, interference between the detector circuit 14 and 24 does not occur and a detection voltage which is higher than a certain level is easily generated. However, since a plurality of detector circuits is required, there is a disadvantage from viewpoints of cost and mounting area.
FIG. 9 is a diagram showing another structure of a conventional transmitter with two transmitting circuits, wherein an equivalent component with that in FIG. 8 is indicated by an identical reference numeral. The transmitter shown in FIG. 9 is different from that shown in FIG. 8 in that, by sharing the detector circuit 14 between the transmitting circuits 1 and 2, the detector circuit 24 is removed. When a detector circuit is shared among a plurality of transmitting circuits in this way, there is an advantage that cost and mounting area are reduced compared with the formulation shown in FIG. 8.
For example, Patent Document 1 describes a formulation wherein a detector circuit is shared among a plurality of transmitting circuits.